High power microwave sources, for example, microwave and millimeter wave high power electronic devices are important in the modern society. These sources/devices have wide applications in, for example, civilian infrastructure and consumer markets (e.g., broadcast media transmission, satellite communication, civilian radar, etc.), military (e.g., electronic countermeasures, high-power weapons, etc.), scientific applications (e.g., plasma heating, particle accelerators, etc.) and industrial applications (e.g., testing and instrumentation, materials processing, etc.).
Vacuum electron devices (VEDs) typically stand out from the solid state devices at high power and high frequencies as VEDs generally exhibit more reliable performance, higher efficiency, and lower cost and weight per watt. Despite the inroads made by solid-state devices, the current business of VEDs is estimated at about 1 billion USD. Among the different types of VEDs, travelling-wave tubes (TWTs) are noteworthy due to their large bandwidth and linearity. Communications satellites, airborne radar systems and unmanned aerial vehicles (UAVs) commonly use travelling-wave tube amplifiers (TWTAs). TWTs have a majority share of 65% in the VED business and it has been projected that such majority share may be maintained in the coming years.
A travelling wave tube (TWT) is one of the most widely used high power microwave devices. The TWT has the largest bandwidth among all microwave vacuum electron devices (VEDs) and it usually acts as a high power amplifier in communication satellites, radar systems and electronic countermeasures (ECM). Apart from the wide bandwidth, TWTs also display the advantages of high efficiency, high linearity, low noise, and high gain in compact packages. The working frequency of TWTs can be from below 1 GHz to hundreds of GHz and are being developed to beyond 1 THz. The output power of normal TWTs may vary between several watts to hundreds of kilowatts. For pulse operation, the peak power of the TWTs may even reach megawatts levels. The efficiency of the TWTs may range from about 30% to about 70%
With relatively minor changes in the operating parameters, a TWT may work as an oscillator instead of an amplifier. Such a device is referred to as a backward-wave oscillator (BWO) and the oscillation frequency may be tuned by varying the acceleration voltage of electrons within the TWT.
The TWT generally includes an electromagnetic waveguide structure. The speed of an electron beam is much slower than the phase velocity in most typical electromagnetic waveguide structures. In order to have “velocity synchronism”, a waveguide structure is needed to slow down the wave speed and such a waveguide structure is referred to as a slow-wave structure (SWS). Commonly used SWSs may include, for example, helix transmission line, coupled cavity, ladder circuits, gratings, helical waveguides and dielectric-lined circuits.
As the operating frequency increases to millimeter wave or terahertz range, the physical dimensions of the various parts of TWTs become smaller and smaller. As a result, traditional fabrication processes can no longer achieve the required accuracy. As a solution to this problem, microfabrication techniques have been proposed in the recent years.
Circular helix slow-wave structure (SWS) has been widely used in travelling-wave tubes (TWTs) due to its wide bandwidth and high coupling impedance. The helix is usually supported by dielectric rods with high thermal conductivity to dissipate the heat from the helix. Normally, the magnetic focusing in the TWT prevents the axially flowing electrons from spreading in the radial direction, but some challenges such as inadequate focusing magnetic field and/or misalignment of the electron gun (e.g., off-axis or inclined with respect to the axis) may cause the electrons to hit the surrounding structure. The charge of the electrons that land on the metallic SWS is conducted away. However, the charge of the electrons that land on the dielectric rods may accumulate on the dielectric rods and may cause a voltage difference between the SWS and the dielectric support material. This voltage may be considerably high to a level causing dielectric breakdown. Even otherwise, a high voltage on the dielectric will affect the electron motion, leading to defocusing of the electron beam and in turn causing more electrons to hit the structure. This problem becomes more severe at millimeter-wave or terahertz frequencies where precise alignment of various parts of the TWT and good control of the magnetic field are more difficult to achieve.
Some methods have been proposed to address the problem of dielectric charging. For example, one method includes coating the dielectric with a thin layer of conductive material. However, the thickness of the coating has been found to be difficult to control and may induce excessive RF loss in the circuits. In another method, coating the dielectric with beryllia has been found to have a relatively less dielectric charging effect. However, the problem of dielectric charging may not be completely avoided in some cases. In yet another method, the material of the rods has been replaced by a lossy dielectric material that exhibits a relatively high electrical conductivity at low frequencies; but this may similarly cause high loss at millimeter-wave frequencies.
As such, there is a need for a SWS that is easily microfabricated and at least minimizes dielectric charging losses, thereby addressing at least the above-mentioned problems.